This invention relates to light-emitting semiconductor devices, or light-emitting diodes (LEDs) according to common parlance, and more specifically to such devices integrally incorporating an overvoltage protector.
Currently attracting the attention of the specialists in the light-emitting semiconductor art are the nitrides capable of generating light in the wavelength range of 365 to 550 nanometers. The devices employing this class of nitride semiconductors, as heretofore made, were mostly capable of resisting electrostatic breakdown only up to 100 volts or so. Attempts have been made to make the devices more immune to electrostatic breakdown. Japanese Unexamined Patent Publication Nos. 10-65215 and 10-200159 are examples. They teach to build an overvoltage protector, which takes the form of a pn-junction diode, on the same sapphire substrate as the LED. The LED and the protector diode are electrically connected reversely in parallel with each other.
In the operation of this prior art overvoltage-protected LED, the protector diode is reverse biased during application of a forward voltage to the LED. The current flows then mostly through the LED and hardly through the protector diode. The protector diode is forward biased, on the other hand, during application of a reverse voltage to the LED. Conduction is then initiated through the protector diode in response to a voltage not less than the predefined conduction-initiating voltage of the protector diode. In that event, therefore, the voltage across the LED is limited to the forward voltage of the protector diode.
This prior art overvoltage-protected LED has proved to possess some inherent shortcomings, however. First of all, it is unnecessarily bulky in size as an inevitable result of the side-by-side arrangement of the LED and the protector diode on the sapphire substrate. The overvoltage-protected LED might be fabricated in the same chip size as the more conventional non-overvoltage-protected LED. But then the LED itself would become so small in size that only a correspondingly less optical output would be obtained. An additional increase in the size of the prior art overvoltage-protected LED results from the need for provision of conductors for interconnecting the juxtaposed LED and protector diode reversely in parallel with each other.
Another drawback arises from the epitaxial growth of the protector diode at the same time with that of the LED according to Japanese Unexamined Patent Publication No. 10-65215, supra. The thickness of the protector diode is therefore subject to that of the LED, making it difficult to set the forward voltage of the protector diode as desired or required. The required forward voltage of the protector diode is higher than the normal reverse voltage of the LED and less than its maximum allowable reverse voltage.
Japanese Unexamined Patent Publication No. 10-65215 also discloses an embodiment in which the LED comprises an active layer of indium gallium nitride sandwiched between two gallium nitride claddings of opposite conductivity types. The protector diode on the other hand comprises a first layer having the same composition as, and joined directly to the n-type cladding of the LED, a second layer of the same composition as the active layer of the LED, and a third layer of the same composition as the p-type cladding of the LED. The cathode on the first layer of the protector diode is electrically connected to the cathode of the LED, which in turn is connected to its n-type cladding. Thus the cathodes of the LED and protector diode are both connected to the same n-type GaN layer. Should they be situated too close to each other, a current might flow therebetween, and no current to the LED. This risk is precludable only by provision of a sufficient space between the cathodes of the LED and the protector diode, a solution objectionable for the resulting increase in the size of the complete device.
The same unexamined Japanese patent publication indicates another embodiment in which a resistor, formed by an extension from the p-type cladding of the active layer of the LED, is connected between the anode and cathode of the LED by way of an overvoltage protector. The cathode of the LED is more or less in Schottky contact with the extension of the p-type cladding of its active layer
This prior art overvoltage-protected LED may therefore be equivalently electrically diagramed as in FIG. 29 of the drawings attached hereto. A Schottky barrier diode SD is therein shown connected via the protector resistor R3 between anode 101 and cathode 102 of the LED. The protector resistor R3 has a value determined by the dimension of the p-type cladding between anode 101 and Schottky barrier diode SD. The Schottky barrier diode SD is connected forwardly in parallel with the LED via the protector resistor R3. As a consequence, if the protector resistor R3 were too low in resistance, a current would not flow therethrough upon application of a forward voltage to the LED. No light would then issue. The sum of the forward resistance R2 of the Schottky barrier diode SD and the resistance of the protector resistor R3 must be sufficiently greater than the forward resistance R1 of the LED in order for the LED to emit light. The sum of the reverse resistance r2 of the Schottky barrier diode SD and the resistance of the protector resistor R3 must be sufficiently less than the reverse resistance r1 of the LED in order for the protector resistor R3 to perform its function of overvoltage protection.
Difficulties have been encountered in creating the extension of the p-type cladding so as to meet the foregoing conditions for the proper operation and functioning of the overvoltage-protected LED. Another objection to the extension of the p-type cladding is that, again, it necessitates an increase in the size of the overvoltage-protected LED.